The present invention relates to an inlet duct for an internal combustion engine that has an air-permeable side wall.
Conventionally known is an intake duct for an internal combustion engine that has a tubular wall formed by a fiber molding made of, for example, non-woven fabric (see, for example, Japanese Patent No. 5091741).
In the intake duct described in the aforementioned document, a downstream duct section made of a hard plastic is coupled to the downstream end of a porous duct made of an air-permeable material such as non-woven fabric. An upstream duct section made of a hard plastic is coupled to the upstream end of the porous duct. The upstream duct section has a funnel-like intake opening.
The intake duct absorbs sound waves of the intake sound when the sound waves pass through the tubular wall of the porous duct. That is, the pressure produced by the sound waves of intake sound vibrates the fibers configuring the tubular wall. The energy generated through such vibration is converted into frictional heat in the fibers and thus consumed. This restricts generation of standing waves of the intake sound in the intake duct, thus decreasing the intake noise.
In the intake duct described in the aforementioned document, the porous duct and the upstream duct section are formed as separate members. This brings about a step in the inner peripheral surface of the section that couples the porous duct and the upstream duct section to each other. The step increases the airflow resistance to the intake air flowing in the intake duct.